A frequency reference oscillator is an important element in various timing and communication devices, such as real time clocks, wireless communications, medical equipment, environment monitoring applications, as well as consumer electronic products such as mobile phones, personal computers and cameras.
Quartz crystal oscillators, which are commonly used as frequency reference oscillators, are incompatible with integrated circuit fabrication. As a result, quartz-based reference oscillators require external circuitry to realize the oscillator, making such oscillators large in size and costly to fabricate.
Micro-Electromechanical System (MEMS) resonators have emerged as an attractive alternative to quartz crystal oscillators. MEMS resonators can be designed into integrated circuits and fabricated together with an integrated circuit. Therefore, MEMS resonators offer comparative advantages of decreased dimensions and lower fabrication cost over quartz crystal oscillators. So far, various types of silicon MEMS resonators have been demonstrated. In particular, a bulk-mode silicon MEMS resonator was characterized to have a high quality factor, for example more than one million. However, bulk-mode silicon MEMS resonators require high direct current (DC) bias voltage, which complicates the integrated circuit design as the voltage used in integrated circuits is well below the required DC bias voltage. Further, the bulk-mode silicon MEMS resonators need to be operated in vacuum, thereby necessitating costly vacuum packaging.
Therefore, there is a need for a new type of resonator that can be realized on an integrated circuit and have a high quality factor, while avoiding the aforementioned drawbacks.